Method for separating in an aqueous medium lanthanides and/or actinides by combined complexing-nanofiltration, and novel complexing agents therefor

ABSTRACT

The invention relates to the separation of lanthanides and actinides by nanofiltration complexation. The object of the invention is to satisfy the existing need for a simple, efficient and economical technique for separating lanthanides and actinides. This object is achieved by a process consisting of using ligands of the polyamino acid type, such as EDTA or DTPA, for complexing lanthanides and/or actinides before separating them by nanofiltration. The invention further relates to novel polyamino acid ligands incorporating ligand structures additional to EDTA and DTPA. Application to the production of rare earths or nuclear waste processing, especially to recycling operations carried out on spent nuclear fuels is also discussed.

This application is a filing under 35 USC 371 of PCT/FR00/01461 filedMay 29, 2000.

FIELD OF THE INVENTION

The invention relates to the separation of rare earths, or lanthanides,and radioactive actinide elements.

In economic and industrial terms, the separation of lanthanidesparticularly concerns the extraction of lanthanides from the orescontaining them, which themselves originate from the earth's crust.Furthermore, lanthanides and actinides are present in the radioactiveeffluents produced by nuclear combustion. The ability selectively toisolate lanthanides and actinides is therefore of crucial economic andindustrial interest.

The present invention therefore relates to a process for separatinglanthanides from one another and/or lanthanides from actinides and/oractinides from one another in an aqueous medium, said process being ofthe type involving complexation-nanofiltration techniques.

The present invention further relates to the application of such aprocess to the production of rare earths and to the reprocessing ofspent nuclear fuel elements containing lanthanides and/or actinides.

The present invention is also a suitable vehicle for publishing novelselective complexing agents for lanthanides and actinides.

DESCRIPTION OF RELATED ART

The earth's crust contains 0.08% by weight of lanthanides. The relativeabundance of lanthanides in ores, which can vary from 50% to 0.01%,makes it difficult to separate the various lanthanides from one another,all the more so because these elements have similar chemical properties.At the present time, the main processes for the production of rareearths involve the hydrometallurgical treatment of enriched ores. Thesetreatments comprise the following steps:

-   -   attacking the ores by a wet process;    -   separating and purifying the solutions obtained by employing        selective precipitation techniques [in the case of rare earths        with an oxidation state other than (III)] and sometimes ion        exchange techniques on resin, but mainly solvent extraction        techniques; and    -   obtaining the finished products (oxides, various salts) or        producing metals by the electrolysis of molten salts at high        temperature or by metallothermics.        By virtue of their particular electronic structure, rare earths        have a large variety of industrial applications: metallurgy,        catalysis, glass, optics, ceramics, electronics, etc.

Actinides derive from the nuclear industry. The potential noxiousness inthe long term (more than three centuries) of nuclear waste originatingfrom the processing-recycling operations carried out on the fuels fromelectrogenic reactors is principally due to the presence of long-livedradionuclides of the elements neptunium (²³⁷Np), americium (²⁴¹⁻²⁴³Am)and curium (²⁴³⁻²⁴⁵Cm); these are called minor actinides as distinctfrom the major actinides uranium and plutonium, which are recycled intothe manufacture of new fuels.

High-activity nuclear effluents contain minor actinides and lanthanidesin oxidation state (III). The lanthanides constitute about one third ofthe fission products and are much more abundant than the minor actinides(=3-5% w/w, based on the lanthanides). It is therefore necessary toseparate them in the knowledge that the choice not to generate secondarysolid waste means selecting, as chemical reagents for the novelprocesses, only substances consisting solely of carbon (C), hydrogen(H), oxygen (O) and nitrogen (N), i.e. substances degradable to gaseswhich can be discharged into the environment when these reagents reachthe end of their life in the processes.

The extraction and separation of the minor actinides, and moreparticularly Am and Cm, in spent nuclear fuel effluents containingfission products rich in rare earths therefore presents a majortechnological challenge.

Following the example of the separation techniques currently employed inthe production of rare earths from ores, precipitation or liquid-liquidextraction techniques are used to isolate the minor actinides fromradioactive effluents rich in lanthanides. These precipitation orliquid-liquid extraction techniques have the major disadvantage ofgenerating waste which then has to be processed. This complicates theprocesses considerably and burdens them with a severe economic handicap.

It is also known to separate the sodium, especially from the cesium, inaqueous effluents originating from the reprocessing of spent nuclearfuel elements. This forms the subject of French patent no. 2 731 831.The process according to said patent consists in reacting theradioactive aqueous effluents containing sodium, among other radioactiveelements, with complexing agents such as ethylenediaminetetraacetic acid(EDTA), polyacrylic acids, polyvinylsulfonic acids, salts of thesepolyacids, a polyethylene-imine or a calixarene of thetetramethylcalix[4]resorcinolarene type. The effluents treated in thisway are then subjected to nanofiltration on a membrane made ofperfluorinated ionomer, polyaramide or porous alumina coated with alayer of sulfonated polysulfone. For the nanofiltration, a pressurevarying from 0.25 to 1.5 MPa is applied to the effluent. The treatedeffluents contain sodium ions, strontium ions and UO₂ ⁺⁺ ions.

With EDTA (Example 7), strontium appears to be the most abundant elementin the retentate, followed by sodium and cesium. Said document does notdeal with the separation of lanthanides or actinides from one another,or lanthanides and actinides.

French patent no. 2 751 336 discloses linear polyphenols used ascomplexing agents in processes of the complexation-nanofiltration typefor separating sodium from cesium in aqueous effluents originating fromthe reprocessing of spent nuclear fuel elements. Here again, saiddocument does not deal with the separation of rare earths and actinides.

The article by GAUBERT et al. published in Sep. Science et Techno.,32(14), 2309-2320, 1997, relates to the separation of cesium fromradioactive effluents by complexation-nanofiltration using a complexingagent of the resorcinarene type.

One is therefore obliged to note that no process is currently availablefor the separation of lanthanides and/or actinides bycomplexation-nanofiltration.

Furthermore, as regards rare earths and minor actinides, it should beemphasized that the industrial protagonists of the technical field inquestion are still waiting for a simple, economic, selective andefficient separation process.

SUMMARY OF THE INVENTION

In this context, one of the essential objects of the present inventionis to provide a process for separating lanthanides from one anotherand/or lanthanides from actinides and/or actinides from one another inan aqueous medium, said process being capable of satisfying the existingneed and overcoming the disadvantages of the separation techniquesinvolving precipitation or liquid—liquid extraction.

Another essential object of the present invention is to provide aprocess for separating lanthanides or actinides from one another and/orlanthanides from actinides, said process being applicable to theproduction of rare earths as well as to the reprocessing of nuclearwaste, especially that originating from the processing-recyclingoperations carried out on spent radioactive nuclear fuels, andparticularly that containing on the one hand long-lived radionuclides,i.e. minor actinides, and on the other hand rare earths.

Another essential object of the invention is to provide novel complexingagents which can be used in separation processes involvingcomplexation-nanofiltration.

DETAILED DESCRIPTION OF THE INVENTION

These and other objects are achieved by the present invention, whichrelates first and foremost to a process for separating lanthanides fromone another and/or lanthanides from actinides and/or actinides from oneanother and/or from other transition metals in an aqueous medium,characterized in that it comprises the following essential steps:

-   -   1—treatment of the aqueous medium with at least one ligand        selected from the group comprising ethylenediaminetetraacetic        acid (EDTA) and/or linear or cyclic polyamino acids, preferably        linear polyamino acids of formula (I) below:        in which:    -   a=0 or 1 and b=2 or 3;    -   c=2 or 3 and d=0 or 1;    -   p=0 to 3, preferably 2;    -   p1=1 to 4, preferably 2 or 3;    -   e=0 or 1;    -   q=1 to 4, preferably 2 or 3;    -   f=2 or 3 and g=0 or 1;    -   h and i, which are identical or different, are each 1, 2 or 3,        preferably 1 or 2;    -   A¹, A² and A³ are identical to or different from one another and        correspond to a monovalent acid group preferably selected from        the group comprising:    -   —COOR, —PO₃R′ and —SO₃R″,    -   where R, R′, R″=H or a cation;    -   the radicals R₁ are identical to or different from one another        and correspond to:    -   Δ H,    -   Δ C₁-C₁₀ alkyl or        where a=0 and R⁹ and R¹⁰ are identical or different and each        correspond to hydrogen or a hydrophilic monovalent radical        preferably selected from amino and/or (poly)hydroxylated and/or        alkoxylated and/or (poly)etherified hydrocarbon radicals, these        radicals preferably being of the (cyclo)alkyl, aralkyl,        alkylaryl, (cyclo)alkenyl, aralkenyl, alkenylaryl or aryl type,    -   R⁹ and R¹⁰ each corresponding even more preferably to a C₁-C₁₀        hydroxy-alkyl, a C₁-C₁₀ alkoxy or a polyol, advantageously a        hydrogenated saccharide;    -   the radicals R² are identical to or different from one another;    -   the radicals R³ are identical to or different from one another;    -   the radicals R⁶ are identical to or different from one another,    -   the radicals R⁷ are identical to or different from one another,    -   R², R³, R⁶ and R⁷ being identical to or different from one        another and corresponding to H or a C₁-C₁₀ alkyl;    -   the radicals R⁴ are identical to or different from one another        and correspond to a hydrophilic divalent group preferably        selected from aromatic amino and/or hydroxylated groups,        aromatic and alkyl amino and/or hydroxylated groups, aromatic        and (cyclo)alkylenic amino and/or hydroxylated groups and        (cyclo)alkylenic amino and/or hydroxylated groups,    -   it being possible for this group to contain alkoxy and/or        (poly)ether radicals,    -   R⁴ preferably being a group        where R¹³ is an amino group and R¹⁴ is a C₁-C₄ alkylene;    -   the divalent group R⁵ is an alkylene, preferably CH₂, or a group        having the same definition as R⁴; or    -   the group R⁸ corresponds to a hydroxyl, to A⁴ having the same        definition as A¹, A² and A³, to hydrogen or to —NR⁹R¹⁰, where R⁹        and R¹⁰ are identical to or different from one another and are a        hydrophilic monovalent radical preferably selected from amino        and/or (poly)hydroxylated and/or alkoxylated and/or        (poly)etherified hydrocarbon radicals, these radicals preferably        being of the (cyclo)alkyl, aralkyl, alkylaryl, (cyclo)alkenyl,        aralkenyl, alkenylaryl or aryl type,    -   R⁸ even more preferably being a C₁-C₁₀ hydroxyalkyl, a C₁-C₁₀        alkoxy or a polyol, advantageously a hydrogenated saccharide;    -   2—(nano)filtration of the aqueous solution treated with the        ligand (I), under a transmembrane pressure greater than or equal        to 0.01 MPa, preferably greater than or equal to 0.1 MPa and        even more preferably of between 0.2 and 1.0 MPa, so as to        collect on the one hand a retentate enriched in at least one        species of lanthanide, actinide or other transition metal, this        species being that which is at least partially complexed with        the ligand (I), and on the other hand a permeate impoverished in        said species; and optionally    -   3—recovery of the ligand/species complexes to be separated in        the retentate, and treatment of these complexes with one or more        appropriate decomplexing agents so as to collect on the one hand        the ligands and on the other hand the target species.

The process according to the invention derives from the combination of aprocess for the selective complexation of actinides and/or lanthanideswith a membrane separation process and more particularly a[nano]filtration process.

It is self-evident that the process according to the invention is notlimited to [nano]filtration in the strict sense, but encompasses anymembrane separation technique which employs a semipermeable membraneforming a barrier between two homogeneous media and offering an unequalresistance to the passage of different constituents of a fluid(suspension, solution, solvent). The force which enables some of saidconstituents to pass through the barrier can result from a pressuregradient (microfiltration, ultrafiltration, nanofiltration, reverseosmosis), a concentration gradient (dialysis) or an electrical potentialgradient (electrodialysis).

The essential advantages of the process according to the invention areits simplicity of implementation, its economic features, its selectivityand the fact that it does not generate secondary solid waste which wouldincur expensive reprocessing. In fact, this process only employssubstances consisting solely of carbon, hydrogen, oxygen and nitrogen.Such organic substances are easily degradable to inoffensive gases whichcan be discharged without adverse environmental repercussions.

It has been possible to achieve these advantageous results by virtue ofthe inventors' worthy and judicious selection of a specific class ofligands which are capable of complexing lanthanides and actinides orother transition metals and whose complexing powers towards variouslanthanides and actinides or other transition metals are sufficientlydifferent to allow a good separation of each of these species from theothers.

The selected ligands/complexing agents are linear or cyclic polyaminoacids, preferably linear polyamino acids, including the well-knowncomplexing agents ethylenediaminetetraacetic acid (EDTA) anddiethylenetriaminepentaacetic acid (DTPA).

In an advantageous mode of implementation of the invention, to separatetwo lanthanides or two actinides or a lanthanide and an actinide orother transition metals in an aqueous medium, preferably an aqueoussolution, one or more ions of the metal(s), lanthanides or actinides tobe separated are subjected to selective complexation.

It is clear that the complexes of ligand/species to be separated whichhave the greatest mass and the greatest steric bulk are those which havethe most difficulty in passing through the nanofiltration membrane underthe effect of the pressure difference prevailing on either side of thismembrane. The non-complexed ions pass through the membrane easily andare therefore separated from the complexed ions in a single step andwithout the use of a solvent.

In step 3, if performed, the complexed ions can be freed or decomplexedafter filtration, for example in a basic medium, by precipitation oftheir hydroxides or by passage over a specific ion exchange resin.Within the framework of this step 3, it is advantageous according to theinvention to make provision for removal of the solvent—in this casewater—for example by evaporation, in order to enable the separated ionsto be recovered.

The equipment needed to carry out the process according to the inventionis relatively limited, the only requirements being a complexationreactor, a pump and a nanofiltration membrane.

To optimize the separation, it is preferable to ensure that there is alarge difference between the complexation constants of thelanthanides/actinides with the ligands of the linear or cyclic polyaminoacid type. This affords a very selective complexation and hence a veryeffective separation.

According to a preferred characteristic of the invention, theligands/complexing agents are linear polyamino acids of formula (I)given above. All the monovalent or divalent groups or radicals referredto in said formula (I) can be linear or branched alkyls or alkenylswhose chain can contain one or more oxygen atoms in place of the carbonatoms (e.g. alkoxy or (poly)ether).

In this same formula (I), “aryl” group is understood as meaning a groupderived from an aromatic hydrocarbon unit containing one or morearomatic rings and capable of being unsubstituted or substituted by OH,alkyl or hydroxyalkyl groups by the removal of a hydrogen atom carbon ofthe ring or by removal of a hydrogen atom from one of the carbons of analkyl or hydroxyalkyl substituent. Examples which may be mentioned arebenzyl alcohol groups or hydroxyalkyl-phenol groups.

Also in this formula (I), “cycloalkylene” group is understood as meaninga divalent group derived from a cyclic hydrocarbon which isunsubstituted or substituted by alkyl or hydroxyalkyl chains by removalof a hydrogen atom, a carbon atom of the ring. An example which may bementioned is the cyclohexylene group.

“Hydrocarbon” is understood in terms of the invention as meaning anygroup containing especially carbon atoms and hydrogen atoms.

Where reference is made to C₁-C₁₀ alkyls, alkoxys or alkenyls, C₂, C₃ orC₄ radicals are more especially intended.

Advantageously, the hydrophilic groups which can correspond to R⁸, R⁹ orR¹⁰ are polyhydroxyalkyls, preferably hydrogenated saccharides and evenmore preferably a sorbitol radical or polyether chains, preferablypolyethylene glycol or polypropylene glycol.

Advantageously, this formula (I) encompasses the well-known linearpolyamino acids EDTA and DTPA (p=0; q=2 or 3; b=2; f=2; e=0; A₁,A₃═COOH; R¹, R⁶, R⁷═H; R⁸═OH).

In one variant, the ligands/complexing agents can be cyclic polyaminoacids such as the DOTAs, which are cyclic polyaminocarboxylates.

One of the essential characteristics of the process according to theinvention is the choice of ligands/complexing agents, especially thoseof formula (I), whose molecular weight is greater than the cut-offthreshold of the nanofiltration membrane, thereby affording a completeretention of the complexed ions.

In a preferred mode of carrying out the invention, the ligand/complexingagent is a product of formula (I.1):

in which R⁹, R¹⁰, R¹¹ and R¹² are identical to or different from oneanother and are each a hydrophilic monovalent radical having the samedefinition as that given for R⁹ and R¹⁰, ethanoyl, methoxyethyl andsorbitoyl radicals being more especially preferred.

It is worth emphasizing that the process according to the inventionmakes it possible not only to separate a given species of lanthanideand/or actinide and/or other transition metal, but also several speciesof these metals. Thus, according to an advantageous provision of theinvention, several metal species belonging to the lanthanide and/oractinide family are separated, said separation being effected bysuccessive complexations of the ions of each of these species to beseparated, the appropriate selective ligand being chosen for eachspecies (step 1) and a nanofiltration (step 2) and adecomplexation/collection (step 3) being carried out after eachcomplexation.

The (nano)filtration membranes usable in the process of the inventioncan be organic, inorganic or organo-inorganic. Their cut-off thresholdmust be such that they allow the non-complexed mono-, di-, tri- andtetravalent ions to pass through and retain the lanthanide or actinideions complexed by the ligands of the invention. The cut-off threshold ofa membrane in respect of a neutral solute can be defined as the minimummolecular weight of a compound which is necessary for a 90% retentionrate of this compound.

According to the invention, the appropriate cut-off threshold for theselected membrane is defined as follows (in g/mol):

100-5000 preferably 200-2000 and even more preferably 500-1500In practice, the cut-off threshold can be e.g. between 200 and 2000g/mol.

These nanofiltration membranes are advantageously made of at least onematerial selected from the group of polymers comprising:

-   -   polyaramides, sulfonated polysulfones, polybenzimidazolones,        grafted or non-grafted polyvinylidene fluorides, polyamides,        cellulose esters, cellulose ethers, perfluorinated ionomers,        associations of these polymers, and copolymers obtained from        monomers of at least two of these polymers.

For further details on nanofiltration membranes, reference may be madeto international patent application PCT WO—92/06675, which describesorgano-inorganic nanofiltration membranes comprising an active layer ofa polymer of the polysulfonated, polybenzenimidazolone, graftedpolyvinylidene fluoride or perfluorinated ionomer (Nafion®) type with acut-off threshold of 300 to 1000 g.mol⁻¹. Reference is also made toFrench patent application no. 2 600 264, which relates toorgano-inorganic membranes comprising an organic porous support and amicroporous membrane made of an organic polymer such as a polysulfone,polyamide, cellulose ester or cellulose ether.

Examples which may be mentioned in particular of preferred membranes forcarrying out the process of the invention are the membranes marketed byOSMONICS under the names SEPA MG-17, SEPA MW-15 and SEPA BQ-01, whichhave a permeability to double-distilled water of between 2 and 101.h⁻¹.m⁻².bar⁻¹ at 25° C.

The separation process on a nanofiltration membrane is preferablycarried out using the tangential filtration technique; this limits thephenomenon of accumulation of the species retained on the surface of themembrane, because the circulation of the retentate causes a strongturbulence in the vicinity of the membrane. Furthermore, this type offiltration enables continuous use.

This can be done using modules in the form of parallel tubes or platessuch as those conventionally employed in this technique. It is alsopossible to use modules in which flat membranes are wound in a spiralaround a hollow perforated tube intended for collecting the permeate.

These modules can be in series and/or in parallel, optionally withdifferent membranes in some of the modules.

The desired separation rates can be obtained by varying the treatmentconditions, such as the pH of the aqueous solution to be treated, thepressure difference, the circulation rate of the retentate and thetemperature used.

The pH of the aqueous solution is preferably in the range from 1 to 7because lanthanide and actinide hydroxides precipitate when the pH isabove 7.

The pH of the solutions to be treated can be adjusted by the addition ofe.g. NaOH or HNO₃.

As far as the temperature is concerned, it is possible to operate atroom temperature or at a lower or higher temperature, for exampleranging from 10 to 40° C.

In the first step of this process, the water-soluble complexing agentconsisting of the derivative of formula (I) is added to the aqueoussolution to be treated. The amount of complexing agent added shall begreater than or equal to one equivalent of complexing agent per atom oflanthanide or actinide to be separated. These complexing agents formcomplexes of the 1:1 type.

For subsequent separation of the lanthanides and/or actinides, theaqueous solution to be treated is circulated in the vicinity of thenanofiltration membrane and a pressure difference is applied between thetwo opposite faces of the membrane in order to collect a permeateimpoverished in lanthanide (or actinide) to be separated, and aretentate enriched in lanthanide (or actinide) to be separated. Thepressure difference between the two opposite faces of the membrane canvary within limits, but good results are obtained with a pressuredifference ranging from 0.2 to 0.8 MPa

According to another of these features, the invention relates to novelcomplexing agents of the polyamino acid type of formula (1):

which:

-   -   a=0 or 1 and b=2 or 3;    -   c=2 or 3 and d=0 or 1;    -   p-=0 to 3, preferably 2;    -   p1=1 to 4, preferably 2 or 3;    -   e=0 or 1;    -   q=1 to 4, preferably 2 or 3;    -   f=2 or 3 and g=0 or 1;    -   h and i, which are identical or different, are each 1, 2 or 3,        preferably 1 or 2;    -   A¹, A² and A³ are identical to or different from one another and        correspond to a monovalent acid group preferably selected from        the group comprising:    -   —COOR, —PO₃R′ and —SO₃R″,    -   where R, R′, R″=H or a cation;    -   the radicals R₁ are identical to or different from one another        and correspond to:    -   Δ H,    -   Δ C₁-C₁₀ alkyl or        where a=0 and R⁹ and R¹⁰ are identical or different and each        correspond to hydrogen or a hydrophilic monovalent radical        preferably selected from amino and/or (poly)hydroxylated and/or        alkoxylated and/or (poly)etherified hydrocarbon radicals, these        radicals preferably being of the (cyclo)alkyl, aralkyl,        alkylaryl, (cyclo)alkenyl, aralkenyl, alkenylaryl or aryl type,    -   R⁹ and R¹⁰ each corresponding even more preferably to a C₁-C₁₀        hydroxy-alkyl, a C₁-C₁₀ alkoxy or a polyol, advantageously a        hydrogenated saccharide;    -   the radicals R² are identical to or different from one another;    -   the radicals R³ are identical to or different from one another;    -   the radicals R⁶ are identical to or different from one another;    -   the radicals R⁷ are identical to or different from one another,    -   R², R³, R⁶ and R⁷ being identical to or different from one        another and corresponding to H or a C₁-C₁₀ alkyl;    -   the radicals R⁴ are identical to or different from one another        and correspond to a hydrophilic divalent group preferably        selected from aromatic amino and/or hydroxylated groups,        aromatic and alkyl amino and/or hydroxylated groups, aromatic        and (cyclo)alkylenic amino and/or hydroxylated groups and        (cyclo)alkylenic amino and/or hydroxylated groups,        it being possible for this group to contain alkoxy and/or        (poly)ether radicals, R⁴ preferably being a group        where R¹³ is an amino group and R¹⁴ is a C₁-C₄ alkylene;    -   the divalent group R⁵ is an alkylene, preferably CH₂, or a group        having the same definition as R⁴; or    -   the group R⁸ corresponds to a hydroxyl, to A⁴ having the same        definition as A¹, A² and A³, to hydrogen or to —NR⁹R¹⁰, where R⁹        and R¹⁰ are identical to or different from one another and are a        hydrophilic monovalent radical preferably selected from amino        and/or (poly)hydroxylated and/or alkoxylated and/or        (poly)etherified hydrocarbon radicals, these radicals preferably        being of the (cyclo)alkyl, aralkyl, alkylaryl, (cyclo)alkenyl,        aralkenyl, alkenylaryl or aryl type,    -   R⁸ even more preferably being a C₁-C₁₀ hydroxyalkyl, a C₁-C₁₀        alkoxy or a polyol, advantageously a hydrogenated saccharide,        with the exception of EDTA and DTPA.

These EDTA and DTPA derivatives (I′) have proved to be excellentcomplexing agents for lanthanides, actinides and transition metals.

These complexing agents (I′) are water-soluble.

Even more preferably, the invention relates to the complexing agents ofthe following formulae:

n being between 1 and 100, preferably between 1 and 10,

Finally, the invention relates to the application of the process and thecomplexing agents (I)/(I′), as defined above, to the production of rareearths or to the processing of nuclear waste, especially thatoriginating from the processing-recycling operations and spent nuclearfuel.

The invention will be understood more clearly from the followingillustrative and non-limiting Examples describing a device for carryingout the process, syntheses of complexing agents/ligands and concretecases of implementation of the process using these complexing agents.

EXAMPLES BRIEF DESCRIPTION OF THE FIGURES

Reference is made hereafter to the attached drawings, in which:

FIG. 1 is an illustration of the device for carrying out the separationprocess according to the invention.

FIGS. 2, 4 and 5 are graphs of the retention rate in % as a function ofthe concentration of complexing agent.

FIG. 3 is a graph of the concentration of Gd and La in the retentate asa function of the ratio diafiltered volume/initial volume.

FIG. 6 to 8 are graphs of the retention rate as a function of the ratioconcentration of complexing agent/concentration of species to beseparated.

BEST MODE OF CARRYING OUT THE INVENTION

FIG. 1 shows an installation of a tangential diafiltration modulecomprising a reservoir 1 containing the effluent (aqueous solutionmedium) 3 to be treated. This reservoir can be maintained at anappropriate temperature by a cryostat 5. The effluent to be treated isled from the reservoir 1 into the filtration module 11 via a line 7equipped with a pump 9. On the one hand the retentate R and on the otherhand the permeate P are withdrawn from this module 11 via the lines 13and 15 respectively. The lines 13 and 15 lead R and P, respectively,into the reservoir 1 and a container 24. The lines 13 and 15 arerespectively provided with a flowmeter 13, and with a conductivity meterand an electrode designated by the common reference 151. The lines 7 and13 are equipped with manometers 17 and 19, respectively, and the line 13is also equipped with a valve 20. A line 21, equipped with a valve 23,connects the part of the line 7 located downstream of the pump to thetop of the reservoir 1. Provision is also made for a line 25 for feedingthe reservoir 1 with water (devoid of species to be separated) as thepermeate P becomes saturated in the container (24). This optimizes thefiltration by recycling he retentate.

The membranes used in the filtration module 11 can be tubular membranesor spiral modules comprising two semipermeable membranes wound in aspiral around a perforated hollow support tube delimiting a tube forcollection of the permeate P. These two membranes are kept anappropriate distance apart by a spacer grid. The filtration membrane isa NANOMAX 5° in the present case.

In the process according to the invention, the complexing agent is addedto the effluent 3, inside the reservoir 1, before starting thecomplexation/filtration process.

Example 1 SYNTHESIS OF DTPA BIS(DIETHANOL)AMIDE (1)

In a 500 ml three-necked flask, 10 grams of DTPA dianhydride (27.98mmol) are dissolved in 150 ml of anhydrous DMF at 80° C. under an inertatmosphere (argon). 17 grams of diethanolamine (167.9 mmol) in 50 ml ofDMF are added dropwise and the reaction medium is stirred for 48 hours.The oily residue obtained is separated from the solvent by decantation.After this residue has been dissolved in the minimum volume of water,800 ml of acetone are added and the viscous precipitate is triturated,isolated from the solvent by decantation and purified on a column ofAmberlite IR-120 ion exchange resin by elution with distilled water.7.98 g of product (1) are obtained in the form of a white powder (50%yield) after evaporation and drying under vacuum.¹H NMR (D₂O): 3.1 (t, J=6.25, 4H); 3.48-3.52 (t+s, 10H); 3.59 (t, J=6.2,4H); 3.76 (t, J=5.2, 8H); 3.91 (s, 4H); 4.49 (s, 4H). ¹³C NMR (D₂O):50.59, 55.88, 56.62, 58.84, 59.95 (CH₂CO₂H and NCH₂CH₂N); 51.75, 52.14,60.85, 61.2 (N(CH₂CH₂OH)₂); 169.02, 172.6, 176.8 (CO₂H and CO). ES-MS:ES*: 566.3 ([M−H1); 282.7 ([M−2H]²⁻]).

Example 2 SYNTHESIS OF THE COPOLYMER DTPA/4,4′-METHYLENE-DIANILINE (2)

In a 250 ml three-necked flask, 1.179 grams of DTPA dianhydride (3.3mmol) are dissolved in 120 ml of anhydrous DMF at 50° C. under an inertatmosphere (argon). 595 milligrams of 4,4′-methylenedianiline (3 mmol)in 45 ml of anhydrous DMF are added dropwise and the reaction medium isstirred for 4 hours at 50° C. The reaction mixture is run into 500 ml ofdiethyl ether. The precipitate is filtered off and washed with 3 times100 ml of diethyl ether. 1.28 g of product (2) are obtained in the formof a white powder (72% yield) after drying under vacuum.

-   -   ¹H NMR (D₂O): 7.17 (s broad, 2H); 6.76 (m, 2H); 3.3-2.1 (m,        14H).    -   Calculation of the degree of polymerization by ¹H NMR:    -   R area of aromatic CH/area of aliphatic CH₂=0.296    -   n=−18R/(20R-8)=2.57

Example 3 SYNTHESIS OF DTPA BIS(DI(2-METHOXYETHYL)AMIDE) (3)

12.4 ml of bis(2-methoxyethyl)amine (0.084 mol) dissolved in 40 ml ofanhydrous DMF are added dropwise to 5 grams of DTPA dianhydride (0.014mol) dissolved in 80 ml of anhydrous DMF under argon at 80° C. Thereaction medium is stirred for 24 hours. After concentration and theaddition of diethyl ether, the oily precipitate is separated from thesolvents by decantation. This residue is dissolved in the minimum volumeof CHCl₃ and reprecipitated in Et₂O. A hygroscopic foam (3) (6.43 g, 74%yield) is obtained after drying under vacuum; it is used without furtherpurification.

¹H NMR (D₂O+NaOD): 2.50 (t, 4H); 2.52 (t, 4H); 2.98 (s, 2H); 3.11 (s,4H); 3.31 (s, 6H); 3.32 (s, 6H); 3.51 (s, 4H); 3.53-3.56 (m, 16H).

¹³C NMR (D₂O): 45.94, 47.03, 47.81, 49.75, 53.69, 56.29, 57.78, 67.16,69.56, 69.69 (CH₂); 58.63, 59.07 (OCH₃); 166.9, 170.5, 175.7 (CO₂H andCO).

ES-MS: ES⁻: 622.1 ([M−H]); ES⁺: 624.3 ([M+H]⁺); 646.2 ([M+Na]⁺).

Example 4 SYNTHESIS OF DTPA BIS(1-DEOXY-1-AMIDOSORBITOL) (4)

In a 500 ml three-necked flask, 5 grams of DTPA dianhydride (0.014 mol)are dissolved in 100 ml of anhydrous DMF at 70° C. under argon. 5.32grams of 1-deoxyl-1-aminosorbitol (0.029 mol) dissolved in 40 ml of DMSOare added dropwise and the reaction medium is stirred for 24 hours. Theviscous reaction residue is separated from the solvents afterdecantation. It is then dissolved in the minimum volume of water andreprecipitated by the addition of acetone. The operation is repeated asecond time and the residual oil is separated off by decantation anddried under vacuum to give a slightly off-white foam (4) (6.66 g, 66%yield).

¹³C NMR (D₂O): 42.17, 47.14, 49.61, 53.41, 56.73, 57.48, 63.16 (CH₂);69.44, 71.08, 71.18, 71.38 (CH); 171.0, 178.86 (CO₂H and CO).

ES-MS: ES⁺: 741.2 ([M+Na]⁺).

Example 5

This Example deals with an aqueous solution containing 2 mmol/l ofgadolinium (Gd) and 2 mmol/l of lanthanum (La) in the form of gadoliniumand lanthanum nitrate hexahydrates. The filtration module used is aplano-spiral module equipped with the NANOMAX 50 membrane marketed byMILLIPORE, which has a surface area A of 0.4 m². The NANOMAX 50plano-spiral membrane has a permeability to double-distilled water of 101.h⁻¹.m⁻².bar⁻¹ at 25° C. A complexing agent consisting of DTPA is addedto the aqueous solution to be treated.

The Gd/La separation is effected under the following conditions:

-   -   transmembrane pressure ΔP=0.5 MPa,    -   temperature=20° C.,    -   retentate flow rate=500 l/h,    -   [NaNO₃]=0.5 mol/l.

Several experiments are carried out with the complexing agent beingadded at concentrations varying from 0 to 2 equivalents of DTPA unitsper atom of gadolinium.

The retention rate is defined by the following formula:RR=[(C ₀ −C _(p))/C ₀]×100where C₀ represents the concentration of the element in the feed andC_(p) represents the concentration of the element in the permeate.

The results of the experiments are given in FIG. 2.

The results in FIG. 2 show that the retention rate of gadolinium isgreater than that of lanthanum when the solution contains between 0 and2 equivalents of DTPA units per atom of gadolinium. The differencebetween the retention rate of gadolinium and that of lanthanum is amaximum when the ratio [complexing agent]/[gadolinium] is equal to 1.This difference then has a value of 14%.

Example 6

This Example deals with an aqueous solution containing 2 mmol/l ofgadolinium (Gd) and 2 mmol/l of lanthanum (La) in the form of gadoliniumand lanthanum nitrate hexahydrates. The filtration module used is aplano-spiral module equipped with the NANOMAX 50 membrane, which has asurface area A of 0.4 m². A complexing agent consisting of DTPA is addedto the aqueous solution to be treated in an amount of 1.1 equivalents ofDTPA units per atom of gadolinium.

The total volume of the solution to be filtered is 4 liters.

The filtration of the solution of Gd and La is effected under thefollowing conditions:

-   -   transmembrane pressure ΔP=0.5 MPa,    -   temperature=20° C.,    -   retentate flow rate=500 l/h,    -   [NaNO₃]=0.5 mol/l,    -   pH=3.8.

This is a diafiltration experiment, i.e. the permeate is withdrawn fromthe reservoir containing the solution to be filtered, and an aqueoussolution containing neither Gd nor La is added simultaneously to thesolution to be treated. This configuration makes it possible to workwith a constant volume in the reservoir containing the solution to befiltered.

The results of the experiments are given in FIG. 3.

The results in FIG. 3 show that the concentration of gadolinium in theretentate decreases much less rapidly than that of lanthanum. When thisexperiment has ended, i.e. when the ratio of the total volumediafiltered to the volume of retentate has reached a value of 20, 88% ofthe initial lanthanum and only 8% of the initial gadolinium have beenremoved.

Example 7

This Example deals with an aqueous solution containing 2 mmol/l ofgadolinium (Gd) and 2 mmol/l of lanthanum (La) in the form of gadoliniumand lanthanum nitrate hexahydrates. The filtration module used is a flatmodule equipped with the SEPA MQ-09 membrane (which has a surface area Aof 0.015 m²). The SEPA MQ-09 flat membrane has a permeability todouble-distilled water of 4.5 1.h⁻¹.m⁻².bar⁻¹ at 25° C. A complexingagent consisting of (3) of Example 3 is added to the aqueous solution tobe treated.

The Gd/La separation is effected under the following conditions;

-   -   transmembrane pressure ΔP=0.6 MPa,    -   temperature=20° C.,    -   retentate flow rate=80 l/h,    -   2<pH<4,    -   [NaNO₃]=0.2 mol/l.

Several experiments are carried out with the complexing agent beingadded at concentrations varying from 0 to 2 equivalents of complexingunits (3) per atom of gadolinium.

The results of the experiments are given in FIG. 4.

The results in FIG. 4 show that the retention rate of gadolinium isgreater than that of lanthanum as soon as complexing agent (3) is addedto the solution to be treated. The difference between the retention rateof gadolinium and that of lanthanum is a maximum when the ratio[complexing agent]/[gadolinium] is equal to 1. This difference then hasa value of 11%.

Example 8

This Example deals with an aqueous solution containing 2 mmol/l ofgadolinium (Gd) and 2 mmol/l of lanthanum (La) in the form of gadoliniumand lanthanum nitrate hexahydrates. The filtration module used is a flatmodule equipped with the SEPA MQ-09 membrane (which has a surface area Aof 0.015 m²). The SEPA MQ-09 flat membrane has a permeability todouble-distilled water of 4.5 1.h⁻¹.m⁻².bar⁻¹ at 25° C. A complexingagent consisting of (2) of Example 2 is added to the aqueous solution tobe treated.

The Gd/La separation is effected under the following conditions:

-   -   transmembrane pressure ΔP=0.6 MPa,    -   temperature=20° C.,    -   retentate flow rate=80 l/h,    -   3<pH<5,    -   [NaNO_(3])=0.1 mol/l.

Several experiments are carried out with the complexing agent beingadded at concentrations varying from 0 to 1.5 equivalents of complexingunits (2) per atom of gadolinium.

The results of the experiments are given in FIG. 5.

The results in FIG. 5 show that the retention rate of gadolinium isgreater than that of lanthanum as soon as complexing agent (2) is addedto the solution to be treated. The difference between the retention rateof gadolinium and that of lanthanum is a maximum when the totalconcentration of complexing agent (2) is 0.7 mmol/l. This differencethen has a value of 21%.

Example 9

This Example deals with an aqueous solution containing 2 mmol/l ofgadolinium (Gd) and 2 mmol/l of lanthanum (La) in the form of gadoliniumand lanthanum nitrate hexahydrates. The filtration module used is a flatmodule equipped with the SEPA MG-17 membrane (which has a surface area Aof 0.015 m) The SEPA MG-17 flat membrane has a permeability todouble-distilled water of 2.5 1.h⁻¹.m⁻².bar⁻¹ at 25° C. A complexingagent consisting of DTPA is added to the aqueous solution to be treated.

The Gd/La separation is effected under the following conditions:

-   -   transmembrane pressure ΔP=0.6 MPa,    -   temperature=20° C.,    -   retentate flow rate=80 l/h,    -   pH=3.8.

Several experiments are carried out with the complexing agent beingadded at concentrations varying from 0 to 2 equivalents of DTPA unitsper atom of gadolinium.

The results of the experiments are given in FIG. 6.

The results in FIG. 6 show that the retention rate of gadolinium isgreater than that of lanthanum when the solution contains between 0 and2 equivalents of DTPA units per atom of gadolinium. The differencebetween the retention rate of gadolinium and that of lanthanum is amaximum when the ratio [complexing agent]/[gadolinium] is equal to 1.This difference then has a value of 45%.

Example 10

This Example deals with an aqueous solution containing 2 mmol/l ofgadolinium (Gd) and 2 mmol/l of lanthanum (La) in the form of gadoliniumand lanthanum nitrate hexahydrates. The filtration module used is a flatmodule equipped with the SEPA MG-17 membrane (which has a surface area Aof 0.015 m²). The SEPA MG-17 flat membrane has a permeability todouble-distilled water of 2.5 1.h⁻¹.m⁻².bar⁻¹ at 25° C. A complexingagent consisting of (1) of Example 1 is added to the aqueous solution tobe treated.

The Gd/La separation is effected under the following conditions:

-   -   transmembrane pressure ΔP=0.6 MPa,    -   temperature=20° C.,    -   retentate flow rate=80 l/h,    -   pH=3.8.

Several experiments are carried out with the complexing agent beingadded at concentrations varying from 0 to 2 equivalents of complexingunits (1) per atom of gadolinium.

The results of the experiments are given in FIG. 7.

The results in FIG. 7 show that the retention rate of gadolinium isgreater than that of lanthanum when the solution contains between 0 and2 equivalents of complexing units (1) per atom of gadolinium. Thedifference between the retention rate of gadolinium and that oflanthanum is a maximum when the ratio [complexing agent]/[gadolinium] isequal to 0.8. This difference then has a value of 82%.

Example 11

This Example deals with an aqueous solution containing 2 mmol/l oflanthanum (La) and 2 mmol/l of uranyl (UO₂) in the form of lanthanum anduranyl nitrate hexahydrates. The filtration module used is a flat moduleequipped with the SEPA MG-17 membrane (which has a surface area A of0.015 m²). The SEPA MG-17 flat membrane has a permeability todouble-distilled water of 2.5 1.h⁻¹.m⁻².bar⁻¹ at 25° C. A complexingagent consisting of (1) of Example 1 is added to the aqueous solution tobe treated.

The La/UO₂ separation is effected under the following conditions:

-   -   transmembrane pressure ΔP=0.6 MPa,    -   temperature=20° C.,    -   retentate flow rate=80 l/h,    -   pH=3.8.

Several experiments are carried out with the complexing agent beingadded at concentrations varying from 0 to 2 equivalents of complexingunits (1) per atom of lanthanum.

The results of the experiments are given in FIG. 8.

The results in FIG. 8 show that the retention rate of lanthanum isgreater than that of uranyl when the solution contains between 0 and 2equivalents of complexing units (1) per atom of lanthanum. Thedifference between the retention rate of lanthanum and that of uranyl isa maximum when the ratio [complexing agent]/[lanthanum] is equal to 1.This difference than has a value of 54%.

Example 12

This Example deals with an aqueous solution containing 1 mmol/l oflanthanum (La) and 36 mmol/l of americium (²⁴¹Am), i.e. a molar ratio[La]/[Am] of 27,777. The filtration module used is a flat moduleequipped with the SEPA MG-17 membrane (which has a surface area A of0.015 m²). The SEPA MG-17 flat membrane has a permeability todouble-distilled water of 2.5 1.h⁻¹.m⁻².bar⁻¹ at 25° C. A complexingagent consisting of (1) of Example 1 is added to the aqueous solution tobe treated.

The La/Am separation is effected under the following conditions:

-   -   transmembrane pressure ΔP=0.35 MPa,    -   temperature=38° C.,    -   retentate flow rate=200 l/h,    -   pH=3.8.

Several experiments are carried out with the complexing agent beingadded at concentrations varying from 0 to 33.3 equivalents of complexingunits (1) per atom of americium.

The results of the experiments are given in the Table below:

Concentration of ligand 0 120 1200 (nmol/l) Ratio 0 3.3 33.3[ligand]/[Am] [La] in permeate (mg/l) 123.6 125 129.4 [Am] in permeate(kBq/l) 1100 900 650The americium 241 was determined by alpha spectrometry. The lanthanumwas determined by ICP-AES. The results in the above Table show that theconcentration of americium in the permeate fractions decreases when theligand (1) is added. When the ratio [complexing agent]/[americium] isequal to 33.3, the retention rate of americium has a value of 41%.

This Example shows that, by adding complexing agent (1), it is possiblegreatly to increase the americium retention without appreciablyinfluencing the lanthanum retention. According to these data, a ratio[ligand]/[Am] in the order of 500 would be necessary to obtain anamericium retention of 92% without appreciably affecting the lanthanumretention (lanthanum retention below 2%).

1. Process for separating lanthanides from one another and/orlanthanides from actinides and/or actinides from one another and/or fromother transition metals in an aqueous medium, comprising the steps of:a) treating of the aqueous medium with at least one ligand selected fromthe group consisting of ethylenediamine-tetraacetic acid, linearpolyamino acids and cyclic polyamino acids; b) (nano)filtering theaqueous solution treated with the at least one ligand through amembrane, under a transmembrane pressure greater than or equal to 0.01MPa, so as to collect a retentate enriched in at least one species oflanthanide, actinide or other transition metal which is at leastpartially complexed with the ligand, and a permeate impoverished in saidspecies; and c) optionally recovering the ligand/species complexes to beseparated from the retentate, and treating the complexes with at leastone decomplexing agent so as to separate the at least one ligand fromthe species.
 2. Process according to claim 1, wherein the at least oneligand is a linear polyamino acid of formula (I):

in which: a=0 or 1 and b=2 or 3; c=2 or 3 and d=0 or 1; p=0 to 3; p1=1to 4; e=0 or 1; q=1 to 4; f=2 or 3 and g=0 or 1; h and i, which areidentical or different, are each 1, 2 or 3; A¹, A² and A³ are identicalto or different from one another and correspond to a monovalent acidgroup selected from the group consisting of: —COOR, —PO₃R′ and —SO₃R″,where R, R′, R″=H or a cation; the radicals R₁ are identical to ordifferent from one another and correspond to: ΔC₁-C₁₀ alkyl or

where a=0 and R⁹ and R¹⁰ are identical or different and each correspondto hydrogen or a hydrophilic monovalent radical selected from the groupconsisting of amino, (poly)hydroxylated, alkoxylated and(poly)etherified hydrocarbon radicals of the (cyclo)alkyl, aralkyl,alkylaryl, (cyclo)alkenyl, aralkenyl, alkenylaryl or aryl type, andmixtures thereof; the radicals R² are identical to or different from oneanother; the radicals R³ are identical to or different from one another;the radicals R⁶ are identical to or different from one another; theradicals R⁷ are identical to or different from one another, R², R³, R⁶and R⁷ being identical to or different from one another andcorresponding to H or a C₁-C₁₀ alkyl; the radicals R⁴ are identical toor different from one another and correspond to a hydrophilic divalentgroup selected from the group consisting of aromatic amino groups,hydroxylated groups, aromatic and alkyl amino and/or hydroxylatedgroups, aromatic and (cyclo)alkylenic amino and/or hydroxylated groupsand (cyclo)alkylenic amino and/or hydroxylated groups, said groupsoptionally containing alkoxy and/or (poly)ether radicals, the divalentgroup R⁵ is an alkylene group or a group having the same definition asR⁴; or the group R⁸ corresponds to a hydroxyl, to A⁴ having the samedefinition as A¹, A² and A³, to hydrogen or to —NR⁹R¹⁰, where R⁹ and R¹⁰are identical to or different from one another and are a hydrophilicmonovalent radical selected from the group consisting of amino,(poly)hydroxylated, alkoxylated and (poly)etherified hydrocarbonradicals and mixtures thereof, the hydrocarbon radicals being of the(cyclo)alkyl, aralkyl, alkylaryl, (cyclo)alkenyl, aralkenyl, alkenylarylor aryl type.
 3. Process according to claim 2, wherein R⁹ and R¹⁰ eachcorresponds to a C₁-C₁₀ hydroxyalkyl, a C₁-C₁₀ alkoxy or a polyol. 4.Process according to claim 3, wherein the polyol is a hydrogenatedsaccharide.
 5. Process according to claim 2, wherein R⁴ is a group

where R¹³ is an amino group and R¹⁴ is a C₁-C₄ alkylene.
 6. Processaccording to claim 2, wherein R⁸ is a C₁—C₁₀ hydroxyalkyl, a C₁—C₁₀alkoxy or a polyol.
 7. Process according to claim 6, wherein the polyolis a hydrogenated saccharide.
 8. Process according to claim 1, whereinthe transmembrane pressure is greater than or equal to 0.1 MPa. 9.Process according to claim 8, wherein the transmembrane pressure isbetween 0.2 and 1.0 MPa.
 10. Process according to claim 1, wherein theions of the metal(s) to be separated are subjected to selectivecomplexation.
 11. Process according to claim 1, wherein the at least oneligand has a molecular weight which is greater than a known cut-offthreshold of the nanofiltration membrane.
 12. Process according to claim1, wherein the at least one ligand is of formula (I.1):

in which R⁹, R¹⁰, R¹¹ and R¹² are identical to or different from oneanother and each is a hydrophilic monovalent radical.
 13. Processaccording to claim 12 wherein the hydrophilic monovalent radicals areselected from the group consisting of ethanoyl, methoxyethyl andsorbitoyl radicals.
 14. Process according to claim 1, wherein severalmetal species belonging to the lanthanide and/or actinide family areseparated, said separation being effected by successive complexations ofthe ions of each of these species to be separated, a selective ligandbeing chosen for each species in step a), a nanofiltration in step b)and a decomplexation/collection in step c) being carried out after eachcomplexation.
 15. Process according to claim 1, wherein thenanofiltration membrane is made of at least one material selected fromthe group of polymers consisting of polyaramides, sulfonatedpolysulfones, polybenzimidazolones, grafted or non-graftedpolyvinyldidene fluorides, polyamides, cellulose esters, celluloseethers, perfluorinated ionomers, associations of these polymers, andcopolymers obtained from monomers of at least two of these polymers. 16.Process according to claim 1, wherein the nanofiltration membrane has acut-off threshold of 100-5000 g/mol.
 17. Process according to claim 16,wherein the cut-off threshold is 200-2000 g/mol.
 18. Process accordingto claim 17, wherein the cut-off threshold is 500-1500 g/mol. 19.Process according to claim 1, wherein said treating takes place in anaqueous medium at a pH between 1 and
 6. 20. Process according to claim1, wherein the aqueous medium treated is derived from spent nuclearfuel.
 21. A complexing agents having one of the formulae:

n being between 1 and 100, and


22. Complexing agent according to claim 21, of formula (I′.2) wherein nis between 1 and 10.